1. Field of the Invention
The present invention relates to an ejector, connected to a fuel cell or the like, for merging fuel discharged from the fuel cell with fuel which is newly supplied, so as to recirculate the fuel. In particular, the present invention relates to a technique for varying the flow rate of the fuel.
2. Description of the Related Art
In conventional solid polymer membrane-type fuel cells, each cell has an anode and a cathode which are provided on either side of a solid polymer electrolyte membrane. A plurality of such cells are stacked so as to form a stack, which is called the “fuel cell” in the following explanations. In the fuel cell, hydrogen, which functions as fuel, is supplied to the anode, while air, which functions as an oxidizing gas, is supplied to the cathode. The catalytic reaction on the anode generates hydrogen ions, and the hydrogen ions are transferred to the cathode via an electrolyte membrane. The transferred hydrogen ions react with oxygen on the cathode, thereby generating electric power.
In order to maintain the ion conductivity of the solid polymer electrolyte membrane, excess water is mixed with the hydrogen which is supplied to the fuel cell, by using a humidifier or the like. Therefore, a specific flow rate of the discharged fuel is set to prevent the buildup of water in the gas passages of the electrodes of the fuel cell.
The discharged fuel is merged with fuel which is newly supplied to the fuel cell, so as to recirculate the fuel. Accordingly, the fuel can be effectively used and the energy efficiency of the solid polymer membrane-type fuel cell can be improved.
Japanese Unexamined Patent Application, First Publication No. Hei 9-213353 discloses an example of such a conventional fuel cell system, in which discharged fuel is recirculated by using an ejector. In the ejector, a second fluid chamber is connected to an opening at the base end (i.e., the end at the base side) of a diffuser which has a tapered inner-peripheral face, and the end of a nozzle, which is arranged coaxially to the diffuser, protrudes into the second fluid chamber; thus, the end of the nozzle faces the opening at the base end of the diffuser. The fuel supplied to the ejector is ejected from the end of the nozzle towards the opening at the base end of the diffuser, so that the discharged fuel supplied to the second fluid chamber is entrained in the above high-speed fuel stream ejected from the nozzle towards the diffuser.
In the above fuel cell system, a pressure gage is provided in a passage for recirculating the discharged fuel. Based on the value detected by the pressure gage, the degree of opening of the fuel supply valve of the ejector is controlled, so that the flow rates of the fuel recharged by the ejector and the newly-supplied fuel can be changed.
A flow meter is provided at the downstream side of the ejector. Based on the value detected by the flow meter, feedback control for the flow rate of the discharged fuel and the newly-supplied fuel is performed, thereby controlling the quantity of the fuel which is consumed in the fuel cell, that is, the output of the fuel cell system.
Japanese Unexamined Patent Application, First Publication No. Hei 8-338398 discloses a variable flow-rate ejector which has a nozzle including a regulating rod which is movable in the axial direction of the nozzle. The open area at the head of the nozzle can be changed by moving the rod in the axial direction by using a driving device (i.e., actuator). According to this function of changing the area of the opening of the head of the nozzle of the variable flow-rate ejector, it is possible to change the index for indicating the efficiency of drawing the fuel from the second fluid chamber into the diffuser, where the index is specifically a ratio of the flow rate Qt of the fuel drawn out from the second fluid chamber into the diffuser to the flow rate Qa of the fuel ejected from the nozzle (i.e., Qt/Qa).
However, when a variable flow-rate ejector employing an orifice whose flow rate is variable is provided in the conventional fuel cell system as explained above, the pressure and flow rate of the fuel must be detected in a plurality of passages in the fuel cell system, so as to perform the feedback control. Therefore, the structure and the control of the fuel cell system are complicated.
In addition, in the mechanism of changing the flow rate by using an actuator as used in the above conventional example, a specific differential pressure between the fuel-supplied electrode and the air-supplied electrode must be controlled with high precision. Therefore, the costs may be increased so as to improve the precision of the actuator, or the size of the actuator may be increased.